Conventionally, there is known a construction machine such as a wheel loader and a working vehicle such as an agricultural machine, an industrial vehicle, and the like provided with a hydrostatic transmission (HST). The working vehicle provided with the HST has such a configuration that a hydraulic pump is caused to communicate with a hydraulic motor via a closed hydraulic circuit and the working vehicle travels by power of the hydraulic motor. Recently, working efficiency can be improved and a fuel can be saved by driving an engine, a hydraulic pump, and a hydraulic motor by an optimum output, pump capacity and motor capacity by an electronic control technology.
As a working vehicle provided with the HST, there is, for example, a working machine which has two hydraulic motors connected in parallel to a hydraulic pump, a clutch is connected to only one hydraulic motor, and power is transmitted to four wheels via a power transmission mechanism such as an axle. The HST is a so-called one-pump/two-motors HST. The one-pump/two-motors HST may be employed when a large hydraulic motor cannot be manufactured or when a location in which a large hydraulic motor is installed is not available on a working vehicle. Further, the one-pump/two-motors HST may be employed because of a reason that when a large hydraulic motor is used, it is difficult to execute prompt response control to inertia due to a rotation of the hydraulic motor and a mechanical resistance also becomes large. In the one-pump/two-motors HST, when a working vehicle travels at a low speed, the working vehicle is driven by two hydraulic motors, whereas when the working vehicle has reached a high speed travel region, the working vehicle travels by power of only one hydraulic motor by releasing a clutch connected to the other one hydraulic motor.